"(...) In the case of Sellistrix, when she sheds taunt, it does so by the game applying a phantom taunt to her. So pretend that you're the OT in vMoL, and she's the Hulk. If you refresh your taunt too early, then when she's "taunted" away, it applies Immunity to her and you need to wait for that Immunity to disappear before your taunt works again (you can see the duration of the Taunt Immunity buff on an enemy if you have a buff tracker that tracks buffs on a target). (...)"
There are several bosses in the new dungeons that will immediately aggro and attack the first player that attacks them, even after being taunted. I've discovered this through very unfortunate trial and error as my group worked on clearing the trifectas. I've learned to drop my AOEs AFTER the tank clearly has aggro in order to avoid being turned on and 1-shot!